Over the longer haul, Congress may intervene, in this case and in a class of similar cases, under the Enforcement Clause of the 14th Amendment, which states:
"Section. 5. The Congress shall have power to enforce, by appropriate legislation, the provisions of this article."
Which provisions? Primarily the Due Process and Equal Protection Clauses of the 14th Amendment:
"No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
I'd say there's a pretty good chance people are trying to deprive Terry of her life without Due Process of law, and Congress has the power to enact civil right legislation to stop States from killing people without Due Process of law.
In the short haul, Blurblogger is quite right to cite Article I Section 8, which states:
"To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."
The power to subpoena is part of, or inherent in, the legislative function of Congress; thus, Congress has the power to Subpoena Terry, and the enforce that Subpoena by preventing Florida from killing her.
A couple spelling/grammatical errors in Post 336, but, um..it's 3:31 AM here on the east coast...
Also -- Terrir is still of an age eligible for militia duty, and therefore her fitness to serve in the militia, or anyone in her condition, is of obvious concern to a Congress operating under the strictest of original intent.